This invention relates to fiber-reinforced composites, and more particularly to fiber-reinforced composites having enhanced surface reflectivity, which may be useful as infrared reflectors having dichroic reflection characteristics. The reflectors may be particularly useful in applications where it is desired to maximize radiant energy output at particular wavelengths, selectively improving heat transfer.
All bodies at a temperature above absolute zero radiate energy. For most bodies at moderate temperatures, the energy radiated falls in the infrared region of the electromagnetic energy spectrum and is commonly called radiant heat. At temperatures where the energy source becomes luminous, the energy output will include the visible portion of the spectrum as well. Active radiant heat sources are readily available and are in wide use. The familiar stem- or fluid-heated radiator may be classified as a dull or low temperature source of radiant energy, as are a great many other widely used heat sources, for example, resistance heaters, radiant heat panels, ceramic-walled space heaters and the like. High temperature or bright energy sources such as ordinary incandescent lamps, vitreous quartz glowbars, electrical resistance elements, halogen lamps, certain forms of industrial lasers and even focussed sunlight are also well known and widely used.
Transfer of energy by radiant means occurs over distances, even in a vacuum. Unless an object is perfectly transparent, a portion of all of the radiant energy incident upon a surface will be absorbed, and a portion reflected. A highly reflective surface will reflect substantially all of the incident radiation, absorbing little energy. Reflectivity of a surface varies with the wavelength of the incident radiation; that is, a surface does not absorb energy equally at all wavelengths. Most non-metallic materials are good absorbers over much of the infrared portion of the spectrum, and many organic materials have principle absorption bands at wavelengths in the portion from about 2 to about 5.5 .mu.m, a band termed by some references the thermal heating band. Radiant heat sources intended for use in toasting or browning are selected to provide a high-energy flux in this portion of the spectrum. In addition, since water and carbon dioxide also absorb strongly at wavelength bands within this region, substances containing high levels of water, for example, latex films, moist solids and the like, may be efficiently dried by using radiant energy in this wavelength region as a means for heating.
Design of radiant heating devices is generally directed toward providing maximum energy output in the thermal region of the energy spectrum in order to use power efficiently. For example, in U.S. Pat. No. 5,157,239 there is described an oven with a grill chamber employing a plurality of halogen lamps as energy sources. At least one of the lamps is provided with a coating selected to reduce the energy output in the visible portion of the spectrum while transmitting the radiant energy output of the lamp in the infrared region. Each source is independently operable, permitting control of the ratio of energy output in the infrared and the visible regions of the spectrum. Radiant energy sources with enhanced output in the thermal infrared regions and at even longer wavelengths are also available. In U.S. Pat. No. 5,077,061 there is described a ceramic heating element capable of efficiently radiating energy at wavelengths of from 3 to 50 .mu.m. These and similar sources provide further opportunity for improving the efficiency of radiant heating devices.
Combining convective heat transfer with radiant heat transfer has been found useful in further improving the efficiency of radiant heat appliances. Although dry air does not absorb significant energy by radiant heat transfer, like other fluids it may absorb heat through direct contact of the air molecules with a heated surface. In convective heat transfer, air flowing over a hotter surface will become heated, and will convey heat to cooler surfaces. A great variety of radiant heating appliances in common use employ a combination of radiant and convective heat transfer modes for space heating. Combined modes of heat transfer are also widely used in processing equipment including baking ovens, driers and the like to improve the efficiency and reduce power consumption. For example, in U.S. Pat. No. 4,333,003 there is disclosed the use of convective air flow over the radiant heat source to improve delivery of heat energy to the surface of a web of material to be dried, or to the surface of a conveyer belt used to carry material through the oven for heat treatment.
Radiant heaters are employed in baking, in the drying of paints or the like, in grilling foods and in space heating. Where a radiant heating device is intended for use in drying or heat-treating a specific substrate, a heat source having enhanced output at particular wavelengths where the substrate absorbs energy may improve the efficiency of the device. In U.S. Pat. No. 5,073,698 there is described a method and apparatus for selectively heating a film on a substrate by providing radiant energy in a band of wavelengths selected to be efficiently absorbed by the film and not by the substrate or carrier. The source of energy is disclosed as a xenon lamp having maximum intensity output in the range 0.8-1.2 .mu.m. According to patentees, matching the wavelength of the energy output to the absorption characteristics of the film reduces or avoids wasting power by not generating energy at wavelengths that are absorbed inefficiently or not at all.
The principle of matching the radiant heat source to the task may be similarly used to improve the design of driers or the like. Water vapor and carbon dioxide absorb energy principally at wavelengths of 2.5-2.8 .mu.m, 4.1-4.5 .mu.m and 5.6-7.6 .mu.m. Sources that radiate energy in those regions may be used to improve the efficiency of equipment used in drying or curing latex films and similar water-containing substrates. Convective air flow devices wherein humid air comprising a normal level of carbon dioxide is heated may also benefit by use of such radiant energy sources. Sources of monochromatic radiation are known. For example, lasers have been used to provide high levels of energy at a very specific wavelength. To be effective, a monochromatic source must be selected carefully to have an output that closely matches the principle energy absorption band that is characteristic of the substrate. Absorption characteristics vary with substrate temperature and with the angle of incidence, and may also be altered by the presence of a surface film and by changes in the composition of the substrate. Lasers are therefore not preferred for general use.
Sources with energy output restricted to a narrow band of wave lengths are also known and these may be more useful as energy sources in heating devices. For example, the device disclosed in U.S. Pat. No. 5,073,698 employs a source that radiates over the narrow range of from 0.8 .mu.m to 1.2 .mu.m, a band selected to be particularly useful for thermal treatment of particular silicon and silicon alloy substrates. Radiant heating devices that will be used with substrates that differ significantly in absorption characteristics may require a plurality of narrow band radiant energy sources, increasing costs and reducing efficiency.
Polychromatic sources radiate a broad band of wavelengths, and may be particularly selected to have a maximum output within a specific region of the energy spectrum. For example, sources are available that radiate over a broad portion of the infrared, from 3 to 50 .mu.m as shown in U.S. Pat. No. 5,077,061. These find wide acceptance for use in devices intended for heating a variety of substrates under a wide range of conditions. However, polychromatic radiant energy sources also suffer disadvantages. Characteristically, a relatively small fraction of the total energy spectrum of such broad band sources falls within one or more of the principle absorption bands characteristic of a particular substrate. A significant portion of the energy output is not efficiently utilized in thermally treating the substrate. Power utilization will therefore be inefficient.
Means for providing a better match between the output spectrum of the source and the absorption spectrum of the substrate that will be heat-treated are needed to effectively improve heat transfer and thereby shorten the time required to carry out the treatment, reducing power usage. For example, where the device will be employed for removing moisture from substrates, augmenting the level of radiation produced in the principle absorption bands associated with water may improve drying efficiency, while improved output within the energy bands characteristic of water and of carbon dioxide could improve radiant heat transfer to moist air and thereby improve the efficiency of devices employing convective air, such as space heaters and the like.